1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device and a production process thereof. More particularly, the present invention relates to an organic EL device, in which a mixed host comprising a mixture of a hole-transporting material and an electron-transporting material is used as a host material of the light-emitting layer, and a production process of such an organic EL device.
2. Description of the Related Art
The organic EL device can exhibit an excellent visibility because light is emitted based on a self-emission mechanism, and also its weight inclusive of the driving circuits can be reduced because the device can be operated at a low driving voltage of from several volts to several ten volts. Accordingly, it is expected to advantageously use the EL device in various applications such as flat-type display devices, illumination devices, backlight devices and others. Furthermore, the EL device is characterized by a variation of colors of the emitted light.
Specifically, the specific characteristics such as high viewing angle, high contrast and low temperature operability of the organic EL device are desirably useful in the application of the device to in-vehicle display devices. However, hitherto, it was very difficult to practice the application of the organic EL device in the production of the in-vehicle display devices, because the EL device has to satisfy the highly increased standards of the qualities for the in-vehicle devices, and also it suffers from the inherent problems due to use of the organic EL materials such as reduction of the luminescence and low heat resistance.
To improve the luminescence durability of the organic EL device, Japanese Unexamined Patent Publication (Kokai) No. 8-48656 teaches formation of the light-emitting layer of the EL device from a mixture of the hole-transporting material and the electron-transporting material as a host material with the light-emitting additive material as a dopant. However, the inventors of this application have found as a result of the study and examination of the proposed EL devices that some of the hole-transporting materials as the host material have a poor heat resistance and thus, if the EL device is used under the conditions of an increased temperature such as 100° C., dark spots and related drawbacks can be remarkably caused in the EL device, thereby resulting in a reduction of the luminescence in the device.
On the other hand, as is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-156290 and Gazette of International Patent Publication WO98/8360, there have been suggested to use a variety of hole-injecting and hole-transporting materials as a host material of the light-emitting layer in the organic EL device. The hole-injecting and hole-transporting materials disclosed therein include the materials having an increased glass transition temperature, thereby enabling to show a good durability at the elevated temperature. Specifically, there have been suggested the specific material containing at least three triphenylamines in a molecule thereof to thereby increase a glass transition temperature of the material to 100° C. or more. However, as a result of the study and examination of the proposed EL devices, the inventors of this application have found that a luminescence durability of the devices cannot be improved as a result of use of such specific material containing triphenylamines, though the generation of dark spots can be inhibited.